In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The photolithography which is currently on widespread use in the art is approaching the essential limit of resolution determined by the wavelength of a light source. As the light source used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp was widely used in 1980's. Reducing the wavelength of exposure light was believed effective as the means for further reducing the feature size. For the mass production process of 64 MB dynamic random access memories (DRAM, processing feature size 0.25 μm or less) in 1990's and later ones, the exposure light source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm.
However, for the fabrication of DRAM with a degree of integration of 256 MB and 1 GB or more requiring a finer patterning technology (processing feature size 0.2 μm or less), a shorter wavelength light source was required. Over a decade, photolithography using ArF excimer laser light (193 nm) has been under active investigation. It was expected at the initial that the ArF lithography would be applied to the fabrication of 180-nm node devices. However, the KrF excimer lithography survived to the mass-scale fabrication of 130-nm node devices. So, the full application of ArF lithography started from the 90-nm node. The ArF lithography combined with a lens having an increased numerical aperture (NA) of 0.9 is considered to comply with 65-nm node devices. For the next 45-nm node devices which required an advancement to reduce the wavelength of exposure light, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are introduced due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the development of F2 lithography was stopped and instead, the ArF immersion lithography was introduced.
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water having a refractive index of 1.44. The partial fill system is compliant with high-speed scanning and when combined with a lens having a NA of 1.3, enables mass production of 45-nm node devices.
One candidate for the 32-nm node lithography is lithography using extreme ultraviolet (EUV) radiation with wavelength 13.5 nm. The EUV lithography has many accumulative problems to be overcome, including increased laser output, increased sensitivity, increased resolution and minimized edge roughness (LER, LWR) of resist film, defect-free MoSi laminate mask, reduced aberration of reflection mirror, and the like.
Another candidate for the 32-nm node lithography is high refractive index liquid immersion lithography. The development of this technology was stopped because LuAG, a high refractive index lens candidate had a low transmittance and the refractive index of liquid did not reach the goal of 1.8.
Under the circumstances, measures for prolonging the ArF immersion lithography were sought. One candidate measure is double patterning. Typical of the double patterning are the self-aligned double patterning (SADP) process of adding film to opposite side walls of lines of a resist pattern and the litho-etch-litho-etch (LELE) process of processing a substrate on every patterning. The SADP process is applied to simple line patterns whereas the LELE process is applied to complex patterns and hole patterns. A combination of SADP with LELE is also contemplated.
It is serious that a resist pattern after development will collapse. In the case of alkaline development, the resist pattern after development is rinsed with water and then spin dried. During spin drying, pattern features are pulled due to interfacial stress, which causes pattern collapse. An attempt to reduce the interfacial stress is made by adding a surfactant to the rinse solution, but is still insufficient to prevent pattern collapse. Another approach for preventing pattern collapse is freeze drying using supercritical carbon dioxide. Since the surface tension of supercritical carbon dioxide liquid is zero, no stresses are applied to the resist pattern during drying of the liquid. However, rinse must be performed in a high-pressure chamber in order to establish a supercritical state. Wafers must be placed one by one into the high-temperature/high-pressure chamber, suggesting that this approach is impractical from the aspect of throughput.